Casey was part of the so-called "golden generation" of Republic of Ireland youth football of the late 1990s. Under the guidance of Brian Kerr, the unfancied Republic won the UEFAU-16 and U-18 European championships in 1998, and Ryan was part of the victorious U-18 side in Cyprus.[2] In 1999, he played at the World Youth Cup in Nigeria, where the Republic reached the last 16 before going out on penalties to the hosts.